This application claims priority to an application entitled xe2x80x9cAPPARATUS AND METHOD FOR MEASURING RESIDUAL STRESSxe2x80x9d, filed with the Korean Industrial Property Office on Mar. 13, 2000 and there duly assigned Ser. No. 2000-12395.
1. Field of the Invention
The present invention generally relates to an optical fiber, and in particular, to an apparatus and method for measuring the residual stress and photoelastic effect of an optical fiber.
2. Description of the Related Art
Optical elements may be subject to various type of strain caused by factors, such as mechanical tension, compression or sheer due to internal stress, and/or improper cooling or annealing during manufacture of the optical element. In particular, the stress generated during the manufacture of an optical fiber, known as residual stress, causes a direct effect in varying the index of refraction in the fiber element. Namely, the residual stress generated in the drawing process of the optical fiber at high temperature causes the refractive index variation by a photoelastic effect and increases the optical losses caused by the light diffusion in the optical fiber.
Accordingly, it is important to measure the residual stress in an optical fiber in order to manufacture high-quality fibers. To this end, a detecting apparatus is used to measure the residual stress associated with the optical fiber. As such, the detecting device is typically provided to exam the spectrum penetrability of the fiber element by observing the light passed through the fiber. In addition, the device detects the distribution of the periodical residual stress along the longitudinal direction of an optical fiber and the refractive index variation caused by the residual stress. Hence, the refractive index variation depends on the direction of a stress remained in the optical fiber and the polarized direction of the light passed through the optical fiber.
When two light beams of orthogonally polarized components penetrate the optical fiber (or optical preform) at a right angle to the axis of the optical fiber (or optical preform), a photoelastic effect is occurred by the residual stress in the fiber which in turn causes a phase difference in the penetrated light according to the polarized direction of the light. Accordingly, the magnitude of the residual stress associated with an optical element can be detected by measuring the phase difference with a polarizer. The following methods uses this phenomena to measure the residual stress along the optical fiber (or optical preform).
One method is disclosed in a thesis paper written by P. L. Chu and T. Whitebread, xe2x80x9cMeasurement of stresses in optical fiber and preform, Appl. Opt. 1982, 21, pp. 4241-4245.xe2x80x9d In this paper, a concrete method and a theory for measuring the residual stress of an optical fiber using a photoelastic effect are suggested. Another thesis paper by J. R. Kropp, xe2x80x9cFast photoelastic stress determination application to monomode fiber and splices, Meas. Sci. Technol. 4, 431-434, 1993xe2x80x9d discloses a similar optical measuring apparatus using the photoelastic effect to observe the residual stress of a mono-mode fiber and the stress generated in the fusion splice of the fiber. Another thesis paper is written by Ken W. Raine, entitled, xe2x80x9cMeasurement of Axial Stress at High Spatial Resolution in Ultraviolet-Exposed Fibersxe2x80x9d and discloses an apparatus for measuring the residual stress of an optical fiber by applying a charge coupled device(CCD) array and a half-shading process.
Another common method for measuring the residual stress of an optical fiber involves examining the residual stress of an optical preform having a larger cross section area (4 cm in diameter) than an optical fiber. However, there is a disadvantage in this method in that a mechanical residual stress generated in the manufacturing process of an optical fiber can not be measured as the optical preform possesses only the thermal residual stress. Thus, measuring the optical fiber directly instead of the optical preform is more preferred. However, the high resolution and micro measurement techniques are required in this preferred method as the size of the optical fiber tend to be relatively small (i.e., cladding diameter=120 xcexcm, core diameter=8 xcexcm). Moreover, when measuring the residual stress of an optical preform, the phase difference of polarized light is expected to be equal to or less than 180xc2x0. However, the optical path length of the optical fiber is only about {fraction (1/100)}th of the optical preform; thus, the phase difference of an optical fiber is estimated but tends to be equal or less than 2xc2x0. Hence, it requires a special measuring device with the resolution of at least 0.1xc2x0. As the diameter of the optical fiber is around 125 xcexcm, the prior system requires an additional magnifying device to project the fine images. In addition, this method has a drawback in that the lens device is not easily and closely attachable to the optical fiber due to the required rotation unit of the polarization analyzer and the one-quarter waveplate.
Furthermore, there is more problem associated with the above prior art system in that both the optical fiber and the background of the optical fiber are modulated due to the periodical variation of the penetrated light when the rotating analyzer is being rotated. As a consequence, this type of modulation makes it difficulty to find the exact image of an optical fiber, thereby causing an image distortion due to the diffusion of light around an optical detector. Accordingly, there is a need for an improved detecting mechanism that can overcome the problems associated with the prior art systems.
It is, therefore, an object of the present invention to provide an apparatus and method for measuring the residual stress and photoelastic effect of an optical fiber in which the distribution of three dimensional residual stress along the longitudinal direction can be measured precisely, thereby improving the manufacturing process of an optical fiber.
Another object of the present invention is to provide an apparatus and method for measuring the residual stress and photoelastic effect of an optical fiber by measuring the light intensity variation penetrated through the optical fiber while gradually increasing a strain over the optical fiber using a fixed polarizer.
To achieve the above objects, there is provided an apparatus for measuring the residual stress and photoelastic effect of an optical fiber and includes: a light source; a rotary type optical diffuser distanced from the light source in a predetermined distance for suppressing the spatial coherence of light radiated in the light source; an optical condenser for condensing the radiated light passed through the optical diffuser into a spot where the optical fiber is located; a polarizer for polarizing the light passing through the optical condenser to a linearly polarized light at an angle of 45 degrees from the axis of the optical fiber; a polarization analyzer installed at an angle of 90 degrees with respect to the polarizer and attached closely to the optical fiber to prevent the polarization error when light pass through the large area of the polarization analyzer; an optical fiber strain unit with a strain sensor for straining the optical fiber on the polarization analyzer along the longitudinal direction, and for measuring the applied strain on the fiber; an object lens for magnifying the image of light penetrated through the optical fiber; and, a charge coupled device (CCD) array for measuring the penetration variation of an optical fiber under the control of an optical fiber strain unit.